Conditionals and counts in SQL - mysql

Question

I have a MySQL table
"ratings", with
an ID column
a column called like_dislike (holds either null, 0, or 1),
and a column called lesson_id (a foreign key from lessons).
the MySQL table, "lessons", with
an ID column
a teacher_id column
I need to select this information:
"SELECT r.like_dislike FROM ratings r INNER JOIN lessons l on l.lesson_id = r.lesson_id";
However, this actually part of a much larger SQL statement, and what I would like to do is:
Foreach lesson_id, if like_dislike == 0, SELECT count(like_dislike) as like
AND
Foreach lesson_id, if like_dislike == 1, SELECT count(like_dislike) as dislike
I do not know how to turn this pseudo code into SQL. I also need to do this in SQL, rather than in something like PHP, because it is part of a larger SQL statement whose conversion into properly formatted arrays is deeply troubling.

Answer 1

You should be able to accomplish this with grouping. For example:
SELECT r.lesson_id, COUNT(*) AS like
FROM ratings r
INNER JOIN lessons l ON l.lesson_id = r.lesson_id
WHERE r.like_dislike = 0
GROUP BY r.lesson_id;
The same for dislike, just change the WHERE clause to
WHERE r.like_dislike = 1
EDIT:
This can be combined into one query as requested, by adding another level of grouping:
SELECT r.lesson_id, r.like_dislike, COUNT(*) AS count
FROM ratings r
INNER JOIN lessons l ON l.lesson_id = r.lesson_id
GROUP BY r.lesson_id, r.like_dislike;
This will give you output, for example:
+-----------+--------------+-------+
| lesson_id | like_dislike | count |
+-----------+--------------+-------+
| 1 | 0 | 12 |
| 1 | 1 | 7 |
| 2 | 0 | 1 |
| 2 | 1 | 4 |
+-----------+--------------+-------+
so for lesson_id of 1, there are 12 likes, and 7 dislikes, etc...
EDIT 2:
To get one row for each lesson_id, you can modify the statement a little:
SELECT r.lesson_id,
CASE WHEN r.like_dislike = 0 THEN COUNT(*) END AS like,
CASE WHEN r.like_dislike = 1 THEN COUNT(*) END AS dislike
FROM ratings r
INNER JOIN lessons l ON l.lesson_id = r.lesson_id
GROUP BY r.lesson_id, r.like_dislike;
For that matter, you don't even need to join on the lessons table at all, unless you are somehow getting ratings that do not link to a lesson. If you want to include lessons that have no ratings, then you will have to change to an OUTER join:
SELECT l.lesson_id,
CASE WHEN r.like_dislike = 0 THEN COUNT(*) END AS like,
CASE WHEN r.like_dislike = 1 THEN COUNT(*) END AS dislike
FROM lessons l
LEFT JOIN ratings r ON r.lesson_id = l.lesson_id
GROUP BY l.lesson_id, r.like_dislike;

Answer 2

Maybe you can use a case statement, something like this:
SELECT r.lesson_id,
case when r.like_dislike == 0 then (count(*)) end as like,
case when r.like_dislike == 1 then (count(*)) end as dislike
FROM ratings r INNER JOIN lessons l ON l.lesson_id = r.lesson_id
GROUP BY r.lesson_id, r.like_dislike
I haven't tested it, but you can see the idea. Further, you must set a case to count 1's and another to count 0's because like_dislike can be null.

Answer 3

try this
SELECT if(r.like_dislike =0 ,count(like_dislike) as like , if( r.like_dislike =1 , count(like_dislike) as dislike, 'its null'))
FROM ratings r
INNER JOIN lessons l on l.lesson_id = r.lesson_id

Answer 4

if you are adding condition on yr datatable then code it like below:
Declare #Counter int
Set #Counter=(SELECT Count(Student_ID) as 'StudentCount' FROM tbCourseSemOne
where Student_ID=1 Having Count(Student_ID) < 6 and Count(Student_ID) > 0)
if(#Counter <6)
print'Sorry! You cannot add more than five subject data for a single stduent'
else
print'Insert Code'
hope it helps